Subaru Pre-Tuning Checklist
Everything you need to know before dyno tuning your Subaru.
18 Things You Should Know Before Tuning Your Subaru
1. Proper Fuel Octane
Fuel octane should be 93 whenever possible, or use the highest fuel octane available locally to you. If traveling to Evans Tuning to get a tune done, and the highest octane available in your area is 92 octane please bring extra fuel with you in 5 gallon containers. We only have 93 within reasonable driving distance to the shop. It’s best to tune on the fuel that is available in your area to avoid potential knock conditions after a custom tune has been performed. If race fuel or e85 is desired to be used for higher power levels, we do stock some VP racing fuels such as C85 and Q16; however, these might not be practical to use if not able to be sourced locally.
2. Colder Heat Range Spark Plugs
For basic bolt on modifications such as 3” turbo back exhaust, intake, and intercooler upgrades, one step colder spark plugs are suggested. The step colder than stock helps with pre ignition, which can lead to knock conditions. It’s not absolutely necessary to have a step colder plugs in for the tuning session, and they can be installed after the tune with no issues. We recommend NGK Iridium spark plugs when possible, but strongly advised that you avoid brands such as Bosch, Auto Lite, etc.
3. Boost Leak Tests
Boost leak testing should be done before the tuning session. Alternatively a smoke test can be performed to visually find a boost leak, but we suggest the boost leak test up to 30-35 psi to verify all the couplers are tight and that there are no leaks on the intake manifold, blow off valve, etc. Having a sealed system after the MAF sensor is vital to do a proper tune. Alternatively, when being tuned via Speed Density (MAF sensor gets ignored, MAP or pressure sensor is used) the system can have a leak and the car will still idle and drive normally. You will, however, lose power/spool from the leak; so, from a power production standpoint it is in your best interest to make sure there are no leaks.
4. Vent To Atmosphere BOV
Vent to atmosphere blow off valves are popular and sound cool; however, they increase the chances of stalling when coming to a stop and rich condition/back firing when lifting throttle. The factory blow off valves are designed fairly well. With stock turbos or drop in turbo applications staying below 22-24 psi level the factory blow off valves do not need to be replaced. If an aftermarket blow off valve is desired, we suggest recirculating it back into the intake like the OEM one. Alternatively, if you are being tuned in Speed Density (no MAF sensor) a vent to atmosphere blow off valve can be used since the MAF sensor is no longer present to monitor any airflow lost in the system when it opens.
5. Fuel Pump Upgrade
At a minimum an upgraded fuel pump is HIGHLY suggested before a custom tune is done. With the normal bolt on modifications (3” turbo back exhaust, intake, intercooler, etc) and the OEM turbo in place the OEM injectors can easily go static, or reach close to 100% operating duty cycle. When this happens the fuel injector can be damaged (stick open or closed), and/or erratic fueling can possibly occur. Ideally the OEM injectors should be operated no higher than 90% duty cycle level. An upgraded fuel pump like DW 65c or a Walbro 255 LPH HP will achieve safe power levels and injector operating conditions.
6. Fuel Injector Upgrade
If an upgraded aftermarket turbo is installed, both fuel injector and fuel pump upgrades are required. The OEM fuel injectors with an upgraded fuel pump can only support around 300~330 whp depending on specific year/model Subaru. We highly recommend Injector Dynamic brand fuel injectors. Once tuned they will run like stock and achieve stock like fuel economy. There are other brand fuel injectors on the market, but they can cause cylinder misfire issues (you’ll get a DTC/check engine light as a result) or poor idle and light throttle driving quality.
7. High Power Fuel System Upgrades
If higher power levels are desired (350 whp and higher…just a simple guideline here), upgrading the fuel rails, lines and fuel pressure regulator are suggested. The OEM lines, rails and regulator can cause uneven distribution at high power levels which can lead to a damaged cylinder.
8. 2008 + STI (GR) Stumble
08’+ Subaru STIs had a redesign on the fuel line/regulator from the earlier generation STIs, which causes a stumble in the 2700~3200 rpm range. Its most commonly referred to as the 'GR Stumble'. In the simplest terms it’s a condition that is caused from a resonance in the fuel system design. Cobb Tuning has a kit available (link to our store) that will alleviate the condition along with a custom tune. The stumble is present in the stock form and some cars exhibit the behavior more pronounced than others. When upgrading fuel injectors the condition almost always rears its ugly head. An alternative to the Cobb kit is upgrading the fuel rails, lines, and fuel pressure regulator to aftermarket versions when pushing higher power levels/boost levels.
9. When Speed Density Is Needed
The MAF sensor on a Subaru with a stock diameter intake tube can support roughly 350~375 whp power level. A larger diameter intake tube can be used to achieve higher power levels, but as the tube diameter grows the MAF reading quality on the low end or idle/light throttle driving suffer. If doing a drop in turbo that supports more than 350~375 whp, we suggest switching over to a Speed Density tune. This allows the MAF sensor to be ignored, or removed altogether. All 02+ turbo Subarus have both a MAP and MAF sensor. The OEM map sensor is good to around 22-23 psi. I prefer not to go higher than 21-22 psi level so that a boost cut can be implemented at 23~23.5 psi. If higher pressure levels are desired, upgrading the map sensor is needed. There are OEM location replacement MAP sensors like Omni 3 (reads up to 28 psi) and 4 (reads up to 42 psi) bar sensors that allow for higher pressure levels to be achieved. If the OEM MAF sensor is removed, an aftermarket air temp sensor needs to be fitted. The most common air temp sensors are GM, which are available from AC Delco or AEM electronics. They are fairly large requiring a 3/8” NPT thread bung to be welded on. There are alternative size GM calibrated air temp sensors in 1/8 NPT thread, which is usually easier fitment considering space constraints. The best location for the air temp sensor is pre throttle body, about 2-6” away.
10. Accessport Firmware Updating
Before coming in for a tune we ask you to make sure your AccessPort hand held is updated with APManager software. We use the latest software from Cobb, which requires that the latest firmware is loaded on the hand held unit. If you are not sure how to do this, we can take care of the update during the tuning session. It ties up 10-15 minutes for the updating process, which we prefer to spend doing the actual tuning on the dyno. In addition, be sure to bring your Accessport unit with you to the tuning appointment.
11. External Wastegate Spring Selection
Externally gated uppipe on pump gas with a stock frame turbo, we recommend running a 12-15psi spring. Running a 12-15 psi spring will allow sufficient control range with an aftermarket 3 port boost solenoid to allow 30-35 psi of boost control. If running a much larger than stock turbo and using e85 for high power levels, a 16-18 psi spring is recommended.
12. Larger Turbos and 3 Port Boost Solenoids
When running a larger than stock turbo, a 3 port boost solenoid is recommended. The OEM boost solenoid does not provide enough control range to prevent boost taper in high rpms, as well as give enough control to achieve higher boost levels needed on a larger than stock turbo.
13. High Flow Cats
High flow cats are not recommended. If possible, it is best to not have one in place as they will brick over time. When this happens, backpressure becomes an issue and can lead to engine damage. Uncatted downpipes are recommended if emissions is not a concern.
14. WRX Intercooler Upgrades
When possible, it is recommended to upgraded your WRX OEM top mount intercooler. When making back to back pulls the stock TMIC quickly heat soaks and will ultimately lead to inconsistent power. The ecu has a knock learn built into the OEM routine, so the ecu will dynamically adjust the ignition timing to keep knock conditions in check. High air temps from a heat soaked OEM top mount intercooler will lead to almost immediate knock conditions.
15. EJ20/EJ25 Oiling Concerns
Check your oil level often! Oil consumption is a major issue on the EJ engine and if you do not check your oil levels often enough the end result could be spun bearings. In addition, it is wise to check the oil pan for dents or to replace the stock pan with a Killer Bee pan/pick up assembly.
16. Front Mount Intercooler Upgrade
A front mount intercooler is recommended for larger than stock sized turbo applications. An upgraded top mount intercooler is a step above the OEM top mount; however the front mount intercoolers provide a much better means of cooling the air charge into the engine as heat soak from the engine bay is not a factor. We have seen some big gains on larger than stock turbos going from an upgraded/larger top mount to a front mount intercooler.
17. WRX Transmission Torque Capacity
The WRX transmission is a weak link. It is important to remain realistic with your power goals and stay at reasonable power levels. The torque capacity seems to be in the 300-350 ft-lb range, but there is no exact level where its safe.
18. Clutch Upgrade
If you are going to be upgrading to a larger that stock turbo, it is a good idea to upgrade your clutch as well. We will tune your car without the upgraded clutch; however, you’ll save yourself the hassle and costs of a retune appointment should you come to your appointment without a clutch that’s properly rated for your power levels. We have seen most STIs and WRXs with normal bolt on modifications (stock turbos) have no issues with the OEM clutch in place while tuning.